Exceptionally omnidirectional broadband light

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received: 30 August 2016 accepted: 17 November 2016 Published: 14 December 2016

Exceptionally omnidirectional broadband light harvesting scheme for multi-junction concentrator solar cells achieved via ZnO nanoneedles Li-Ko Yeh1,2, Wei-Cheng Tian2, Kun-Yu Lai3 & Jr-Hau He1 GaInP/GaAs/Ge triple-junction concentrator solar cells with significant efficiency enhancement were demonstrated with antireflective ZnO nanoneedles. The novel nanostructure was attained with a Zn(NO3)2-based solution containing vitamin C. Under one sun AM 1.5G solar spectrum, conversion efficiency of the triple-junction device was improved by 23.7% via broadband improvement in shortcircuit currents of 3 sub-cells after the coverage by the nanoneedles with a graded refractive index profile. The efficiency enhancement further went up to 45.8% at 100 suns. The performance boost through the nanoneedles also became increasingly pronounced in the conditions of high incident angles and the cloudy weather, e.g. 220.0% of efficiency enhancement was observed at the incident angle of 60°. These results were attributed to the exceptional broadband omnidirectionality of the antireflective nanoneedles. Multi-junction solar cells hold great promise in the photovoltaic industry because of the high power conversion efficiencies (η’s) that are not achievable with their single-junction counterparts1–3. In particular, the multi-junction solar cell equipped with a concentrator system has delivered the record high η4. Since concentrator solar cells utilize focusing lens or tracking mirrors to collect sunlight in large areas, the output power generated by a unit cell area can be increased, leading to the reduced cost of cell materials5. To improve photovoltaic performances of the multi-junction devices, suppressing the undesired surface reflection, particularly at increased incident angles, is of crucial importance. In a solar concentration system, it is extremely difficult to avoid the deviation angle relative to the aligned optical path even with the state-of-the-art tracking apparatus6,7. The deviated light beams can lead to undesired efficiency loss owing to the low absorption at high incident angles8,9. One way to address the issue is to increase the omnidirectionality of reflection suppression on device surface. The antireflective (AR) surface with enhanced omnidirectionality can trap the photons from various incident directions through the scattering effect, in which light diffuse reflection is induced by the mircostructures or nanostructures properly fabricated on the surface10–17. The so-called light trapping effect greatly prolongs the optical path along the air/device interface, increasing the chance of optical absorption by the active region11. Moreover, the induced light trapping effect can also benefits solar cells via the improved irradiance uniformity, which is strongly desired by the concentrator. The concentrated solar irradiance can easily damage the device by overheating the junctions18. Since the diffuse reflection triggered on the mircostructured or nanostructured surface can trap and redirect the incoming photons, the very high solar irradiance should be more evenly distributed on the device surface, preventing the thermal damage in localized areas. Recently, with the advances in growth/fabrication techniques, the feasibility of omnidirectional and broadband suppression of surface reflection has been reported with many types of AR nanostructures19–21, bringing 1

Computer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. 2Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC. 3Department of Optics and Photonics, National Central University, Chung-Li 32001, Taiwan, ROC. Correspondence and requests for materials should be addressed to K.-Y.L. (email: [email protected]) or J.-H.H. (email: [email protected]) Scientific Reports | 6:39134 | DOI: 10.1038/srep39134

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Figure 1. (a) Layer structure of the triple-junction solar cell. (BSF: back surface field; TJ: tunneling junction). The red dash-line rectangle encloses the ZnO AR nanostructure, which are displayed by the cross-sectional SEM images in (b) the nanorods grown without vitamin C; (c) the nanoneedles gown with vitamin C. promising potential for single- and multi-junction solar cells22,23. The enhanced omnidirectionality of these nanostructured AR coating can further slash the cost of concentrator solar cells by making them less dependent on the pricy tracking system7,8. In addition to the aforementioned light trapping effect, the excellent AR properties of nanostructures also come from her capability of suppressing the reflection at long wavelengths. Since the nanoscale features become less resolvable in the long-wavelength solar spectrum, the nanostructured surface behaves as a transition layer from air to the device, breaking the abrupt transition of refractive index and thus facilitating optical transmission through the interface24. In general, increasing the grading of refractive index at the interface should render increased light absorption, but shaping the nanostructure with a certain desired feature proved to challenging. There have been numerous studies demonstrating superior photovoltaic performances of the devices with nano-engineered AR coating25–27. For example, it has been shown that GaAs single-junction solar cells covered with syringe-like ZnO nanorod arrays exhibit a 30% enhancement in conversion efficiency25. However, the results on multi-junction solar cells are much less found, let alone the studies with specifically shaped nanostructures and the characterizations under concentrated solar intensities and angle-dependent measurement. In this work, a novel ZnO AR nanostructure is applied to commercial GaInP/GaAs/Ge triple-junction solar cells. The needle-like nanostructure is synthesized using a cost effective hydrothermal method, in which the diameter of a single rod can be gradually shrunk from the bottom to the top. Covered by the unique tapered nanorods, ηof the tandem solar cell is effectively improved by 23.7% under normal incidence due to broadband improvement in short-circuit currents of all of 3 sub-cells. As the incident angle increases to 60°, the η enhancement goes up to 220%. Moreover, under the illumination of 100 suns, ηof the tandem solar cell coated with the ZnO nanoneedles is 45.8% higher than that with bare surface. The excellent omnidirectionality and heat sustainability demonstrated here bring promising potential for multi-junction concentrator solar cells.

Results

The ZnO nanostructures synthesized by the hydrothermal process are displayed by the scanning electron microscopy (SEM) images, as shown in the dotted line section of Fig. 1(a). Figure 1(b) and (c) present the nanorods grown without and with the addition of vitamin C, respectively. Comparing the two figures, it can be seen that Scientific Reports | 6:39134 | DOI: 10.1038/srep39134

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Figure 2. (a) Specular reflectance measured on the solar cells with different surface conditions. The inset is transmittance measured with the glass covered by ZnO nanorods and nanoneedls. (b) AOD-dependent reflectance on the devices with incident angle and wavelength fixed at 30° and 550 nm, respectively.

adding vitamin C in the hydrothermal process leads to the tapered ends of the nanorods, exhibiting a needle-like feature. The tapered ends are attributed to the adhesion of vitamin C in the top region of the nanorods, which prevents the reaction of precursor in the proximity and thus results in gradual decrease in vertical/lateral growth rates of the nanorod25. The average length of the nanorods (Fig. 1(b)) and the nanoneedles (Fig. 1(c)) are both around 1.2 μm, and the base diameters seen in the two figures are all less than 100 nm. Since ZnO has a nearly-zero extinction coefficient and the refractive index of around 2.02 (an intermediate between those of air and the GaAs contact layer)27, the oxide nanostructure is expected to act as an effective AR medium. Figure 2(a) shows the spectra of specular reflectance (incident angle: 5°) measured on the solar cells with different surface conditions: the bare surface, the nanorods and the nanoneedles. The inset compares the transmittance of nanorod- and nanoneedle-covered glass slides. These measurements were performed with a standard UV-VIS spectrometer (JASCO ARN-733) equipped with an integrating sphere. One can see spectra oscillation occurring at the wavelengths above 700 nm, which is due to the fact that the device layer structure and the nanostructures are less resolved by the long wavelengths, and therefore interferences take place as light beams reflected at the layer interfaces. Evidently, the reflectance is considerably suppressed on the nanostructured surface. The suppressed reflectance mainly comes from the nanoscale dimension of the AR coating. Since the geometric features of the nanorods and the nanoneedles are mostly smaller than the studied wavelengths, the incident wave sees the nanostructure as an effective medium whose effective refractive index falls in between those of air and ZnO25. The effective medium provides a graded refractive index because of the increased space filling factor of ZnO toward the bottom, significantly reducing the undesired reflectance through destructive interferences among the beams reflected from different depths into the nanostructure19. As the nanoneedle arrays exhibit the most index grading from air to the device, the lowest reflectance (as well as the highest transmittance) is achieved. The excellent AR property of the nanoneedles is also manifested at high-order scattering angles, which is demonstrated in Fig. 2(b). The figure presents the reflectance measured at the angle of detection (AOD, defined as the angle between surface normal and the reflected beam) ranging from 5° to 60°, with the incident angle fixed at 30°. The incident wavelength is selected to be 550 nm, being close to the peak of AM 1.5G solar spectrum power. It is obvious that the nanoneedles not only suppress the specular reflectance at 30°, but also render the greatly reduced reflectance at other scattering angles. One can further find that the extent of reflectance reduction becomes greater at increased AOD, indicating the exceptional light trapping capability of the nanoneedles12. The results shown in Fig. 2 suggest that the nanoneedles can capture much increased photons for the solar cell operating at high incident angles. Figure 3(a) shows the current density–voltage (J–V) curves under the solar illumination of one sun AM 1.5G (100 mW/cm2). Device characteristics derived from the curves are summarized in Table 1. It can be seen that the nanostructured AR coatings result in enhanced short-circuit current densities (Jsc’s), indicating that the facilitated optical transmission into the device generates additional photocarriers. One can also see that covering the solar cells with ZnO nanorods or nanoneedles does not sacrifice the fill factor, which tells that surface passivation of the device is well maintained with the AR nanostructures. Overall, the enhanced Jsc brought by the nanoneedles without sacrificing other photovoltaic performances boosts the conversion efficiency of the solar cell by 23.7%, i.e. from 22.4% (of the bare surface) to 27.7%. Although the absolute efficiency of the triple-junction solar cell is not as high as other commercial products, which can be due to the unoptimized material qualities and fabrication conditions, the efficiency enhancement seen in Fig. 3(a) clearly shows that the ZnO nanostructure is effective in boosting the photovoltaic performances of multi-junction devices. The effect of the nanostructures on photocurrent generation by each sub-cell can be analyzed with the external quantum efficiency (EQE) spectra, as presented in Fig. 3(b). The EQE spectra in wide wavelength range unambiguously demonstrate that the nanostructured AR coatings benefit carrier generation of all the three sub-cells. For example, the nanoneedles significantly raise the EQE at λ = 940 nm to 63.8% for the Ge bottom cell, which is 59% higher than that (40.2%) attained with the bare surface. Table 2 lists the short-circuit current densities of each sub-cell calculated by the following equation:

Scientific Reports | 6:39134 | DOI: 10.1038/srep39134

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Figure 3. (a) J-V curves under AM 1.5G solar spectrum and (b) EQE spectra measured on the solar cells with the surface conditions of bare, nanorods, and nanoneedles.

AR Layers

Voc (V)

Jsc (mA/cm2)

Fill Factor

η (%)

Bare

2.35

11.32

0.84

22.4

Nanorods

2.39

13.32

0.83

26.6

Nanoneedles

2.40

13.82

0.83

27.7

Table 1. Device characteristics extracted from the J-V curves in Fig. 3(a).

AR Layers

Top cell (mA/cm2)

Middle cell (mA/cm2)

Bottom cell (mA/cm2)

Bare

10.82

9.44

11.02

Nanorods

11.97

10.18

13.72

Nanoneedles

12.76

10.58

14.82

Table 2. The photocurrents of each sub-cell extracted from the EQE spectra in Fig. 3(b).

J SC = (q /hc )

∫ φ (λ) ⋅ EQE (λ) dλ

(1)

where φ(λ) is the intensity of AM 1.5G spectrum, EQE(λ) is the measured EQE obtained in Fig. 3(b), q is the electronic charge, λis the wavelength of the monochromatic light, h is Planck constant, and c is the speed of light. It is found that the Jsc generated by the middle cell is lower than the Jsc listed in Table 1, which can be due to the variation of intensity distribution in the AM 1.5G spectrum used in the measurement. The results of Fig. 3(b) and Table 2 highlight an essential trait required by the AR coating for multi-junction solar cells: the wavelength range of reflection suppression must be broad enough to cover all the working ranges of each sub-cell. Since the Jsc under sunlight is limited by the cell delivering the least photocurrent, the AR structure merely promoting the EQE of a certain sub-cell does not promise the eventual η enhancement28. In other words, if only one or two of the three sub-cells shows promoted EQEs, the conversion efficiency under solar spectrum may still remain unchanged. As shown in Fig. 2(b), the increased index-grading of nanoneedles contributes to the most suppressed reflection over the studied wavelength range (350–1500 nm), in which EQE boosts are observed for all the three sub-cells, therefore a considerable ηenhancement is achieved. A close observation leads to the finding that EQE enhancement due to the nanostructure becomes less pronounced at the absorption wavelengths of the middle cell, such as λ = 730–750 nm and λ = 830–860 nm. The result should be discussed in consideration of the reflectance spectra shown in Fig. 2(a). Comparing the EQE and the reflectance spectra measured on the nanostructured surface, one can find that the wavelength ranges with less EQE enhancement correspond to those with interference oscillation peaks in the reflectance spectra, i.e. the EQE local minima occur at the wavelengths corresponding to reflectance local maxima. The reflectance peaks of the nanorods or nanoneedles compromise their AR performances, and thus result in the damped EQEs seen in Fig. 3(b). The results of Fig. 3(b) confirm that the superior broadband reflectance suppression by the nanoneedles is beneficial for the light harvesting of each sub-cell. Figure 4(a) presents the η’s acquired with different incident angles. The measurement was carried out under one sun AM 1.5G solar simulator and the incident angle (θ) is that between the surface normal and the incident light beam, i.e. θ = 0° corresponds to normal incidence. One can see that η’s of the devices with nanorods or nanoneedles are apparently larger than that of the bare device at all θ’s. In particular, the η’s measured with the

Scientific Reports | 6:39134 | DOI: 10.1038/srep39134

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Figure 4. Conversion efficiencies of the triple-junction solar cells measured under AM 1.5G illumination as a function of (a) incident angle (under one sun); (b) solar concentrator ratio. The inset in (a) is ηratio of the nanoneedle device (ηNN) to the bare one (ηbare) plotted as a function of incident angle.

nanoneedles are fairly maintained at −30°